Nicotinamide adenine dinucleotide (NAD+) is the parent compound of the pyridine nucleotide family of coenzymes (NADH, NADP, NADPH) that act as essential cofactors and electron transporters in a number of metabolic processes including alcohol, lactate and amino acid metabolism and energy (ATP) production. NAD+ is an essential substrate for a number of important NAD-dependent enzymes including Poly(ADP-ribose) polymerase (PARP), Sir2-homolog (SIRT1) and NAD glycohydrolase (CD38+). PARP is a nuclear enzyme which mediates repair of DNA double or single strand breaks, while the NAD-dependent deacetylase SIRT1 affects gene silencing and cellular longevity. NAD glycohydrolase catalyzes the production of cyclic ADP-ribose (cADPR) from NAD+ and affects intracellular calcium signalling. The role of NAD+ in these and other cellular functions suggest that in addition to being a regulator of metabolic activity, NAD+ plays a central role in the control of fundamental cellular processes.
Maintenance of NAD+ levels within both the cytoplasm and nucleus is therefore vital to sustaining nuclear integrity, cell viability and growth. Reduced levels of cellular NAD+ consistently correlate with death in a number of cell types, and are prevalent in degenerative disorders associated with oxidative stress. Oxidative stress is characterised by the presence of excess oxidative compounds that may induce oxidative stress and/or damage, for example, reactive oxygen species (ROS), superoxide radical, hydroxyl radical, nitric oxide, ozone, thiyl radicals, and carbon-centred radicals (e.g., trichloromethyl radical). ROS such as H2O2, O2.—, .OH and NO, have detrimental effects including inactivation of specific enzymes via oxidation of their co-factors, oxidation of polydesaturated fatty acids in lipids, oxidation of amino acids within proteins and DNA damage. Oxygen free radical activity is responsible for several important molecular cascades that underlie a number of pathologic processes including neurodegeneration, ischemia-reperfusion injury, atherosclerosis, inflammation, DNA damage in skin cells (e.g. keratinocytes and fibroblasts) and potentially tumor generation through deregulated cell signaling following DNA damage.
Increased ROS production is known to result from exposure to U.V. or ionising radiation (x-ray, γ-rays), chemical agents, infection, inflammation or reduced mitochondrial efficiency. Any one or combination of these factors may be prevalent in chronic degenerative states such as normal cellular aging, accelerated aging of the skin, Alzheimer's disease and Parkinson's disease, as well as chronic or acute UV induced damage in skin cells. Alzheimer's and Parkinson's diseases are examples of neurodegenerative disorders characterised by progressive loss of neuronal cells. The primary cause of brain cell death is not known but appears to be mediated by inflammatory changes associated with oxidative stress and accelerated DNA damage.
DNA strand breaks caused by oxidative damage require a rapid repair response which uses up the cells vital NAD+ resource. While NAD+ serves as a substrate for a number of enzymes, the most significant contributor to rapid NAD+ turnover and depletion is activation of Poly(ADP-ribose) polymerase (PARP) enzyme family members, in particular PARP-1. As mentioned previously, PARP-1 is a DNA binding enzyme activated by double or single stranded breaks to the DNA and is critical to the base excision repair (BER) process. Although many proteins are involved in repairing DNA damage the majority of DNA lesions are repaired by BER. Accordingly, NAD+ plays a central role in DNA repair and intracellular levels are rapidly reduced during oxidative stress. Improving antioxidant capacity and DNA repair is therefore a mechanism by which cell viability may be promoted and retained.
It is clear that maintaining NAD+ levels within both the cytoplasm and nucleus is vital to sustaining nuclear integrity, cell viability and growth. Accordingly, there is a general need for treatments capable of increasing cellular levels of NAD+ in circumstances where NAD+ is depleted. Therapies for the treatment of oxidative stress have largely focused on the prevention of free radical production (chelation therapy) or the reduction of molecular damage (antioxidant therapy). Such treatments do not provide a means of alleviating DNA damage caused by ROS, and thus have limited potential in reducing cell death and deregulated cell growth (i.e. tumour proliferation). Hence, there is a need for treatments capable of enhancing DNA repair in diseases and conditions associated with chronic or acute oxidative stress.